One method of determining the attenuation in wave guides has been by launching light into fiber from one end of the fiber and measuring the output power of light at the opposite end of the fiber. A second measurement is made after cutting a short length from the input end of the fiber. The attenuation is then calculated from the two power measurements made and the length of fiber that is removed between the successive power measurements. A modification of this first method couples light from a laser into the glass fiber by way of a taper coupler; the fiber being heated and necked down at the point of coupling. A portion of the fiber is cut off, as in the first described method.
Another method uses a quick-connect-disconnect splice joint where the output power is a function of the fiber attenuation plus the loss at the joint. The coupled power is obtained by attaching a detector in the joint in place of a fiber. This method suffers from lack of inherent repeatability because of variations in position in mechanical connect-disconnect connectors and variations in geometry of fibers under measurement (varying coupling) and consequently results are not uniformly accurate.
The present invention does not require destruction of any portion of the wave guide, and the method has a light source of desired wavelength, preferably collimated, which can be passed through a collimating beam expander with or without spatial filter. The expanded light beam is passed through a beam splitter. The split beams may then be reflected by right angle prisms or mirrors, preferably through neutral density filters, and the two parallel light beams of equal or constant power ratio are passed through converging lenses and launched in opposite directions along the length of the fiber and preferably focussed on the same point at the axis of the fiber. Dove prisms are preferably used with a groove in one of the prisms to maintain the fiber in a straight line at the location where the light is launched into the fiber.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.